Solar powered UAVs have been proposed for use as long endurance aerial platforms, flying in the generally benign conditions of the stratosphere (typically at altitudes between 15,000 and 30,000 m), for such roles as communications relay, earth observation, weather monitoring, surveillance and mapping. In this respect they may provide a cost-effective alternative to the earth-orbital satellites conventionally used for these purposes and have the additional advantage that they can be easily controlled to remain in sight of a selected part of the globe, by circling or following some other suitable flight pattern, or to fly to any other desired station. Such proposed aircraft typically comprise a lightweight structure including at least a mainplane, one or more electrically-driven thrust-producing motors, an array of photovoltaic cells, and electric storage means such as rechargeable batteries or regenerative fuel cells. Examples of such aircraft are described in U.S. Pat. No. 5,810,284 and WO2004/106156. In principle aircraft of this kind can be capable of remaining aloft indefinitely, i.e. so-called “perpetual flight”, limited only by the integrity of their airframe, power and control systems and mechanical components, by following a diurnal flight pattern in which, during the hours of daylight, the power generated by the photovoltaic cells is used to drive the motors at high power to climb the aircraft from a base operational altitude to a higher operational altitude and to charge the electric storage means and, during the hours of darkness, the motors are driven at lower power from the storage means and the aircraft is allowed to descend to the base (though still stratospheric) operational altitude, and so on. Alternatively the aircraft may remain at a substantially constant operational altitude throughout both the day and night if its electric storage means are of sufficient capacity to maintain altitude during the hours of darkness.
It will be appreciated that to make best use of the finite power resources available for such an aircraft it should be as aerodynamically efficient as possible while avoiding or minimising any consequent weight penalty. For this reason the invention proposes the use of a mainplane with canted down tips, generally in accordance with the teachings of WO2006/030213. Tips of this type have been found to achieve a reduction of induced drag and help to relieve wing root bending moment. The latter effect is particularly enhanced when the tip is also raked back in planform, featuring a leading edge sweep and trailing edge sweep both increasing, and a chord reducing, in the outboard direction of the device.